The present invention relates generally to the field of fiber reinforced composites and, more particularly, to an apparatus and method for manufacturing fiber reinforced composite products having a substantially closed, hollow interior, such as wheel rims, from such materials.
A composite material is a combination of two materials which has its own distinctive structural properties. One well known type of composite, fiber glass reinforced plastic (FRP), is formed using filaments or woven cloth of fiber glass embedded in a polyester or epoxy resin base. Fiber glass reinforced plastic composites may lack the stiffness of other polymer fiber filaments such as KEVLAR.RTM., SPECTRA.RTM., carbon, ceramic and boron to name a few. Composites made from such relatively high strength reinforcing polymeric fibers are generally known within the industry as advanced composites.
The use of advanced composites has in the past been restricted to relatively high technology military and aerospace applications. More recently, however, advanced composites have begun to be used outside these areas and their use is now expanding widely in areas such as the automotive and sports industries.
One example of the use of advanced composites in the sports industry is in bicycle racing. Like cars and airplanes, the main factor inhibiting speed for a bicycle is wind resistance. Wind resistance accounts for up to 90 per cent of the total force slowing down a cylist. Especially in racing situations, it is important to reduce wind resistance to the cyclist as much as possible. Because advanced composites have relatively high strength to weight ratios as compared to previously used metals such as steel and aluminum, manufacturers of racing bikes are using advanced composites wherever possible. For instance, the bike frames used by the U.S. team in the 1988 Olympics are made of advanced composite tubes weighing 3.3 lb. Each tube is shaped like an airfoil to reduce drag, while being stiffer and lighter than steel.
Some hollow structural members, such as a bicycle frame, require the presence of openings. In a bicycle frame openings are required in the frame for the handlebar, seat and pedal assemblies. Since openings are needed anyway, it is a relatively easy task to insert air into the tube openings to form the hollow frame using conventional advanced composites molding techniques.
However, other products have shapes which do not lend themselves well to forming by such techniques. In the case of a bicycle wheel rim, the overall strength of the rim is only as great as that which exists at its weakest point. Therefore it is desirable that a wheel rim not exhibit any structural weak points. Thus, while making the interior hollow to save weight and material is desirable for a composite rim, openings to the hollow interior are not desired, especially when such openings would create discontinuities in the fiber reinforcement. Previous conventionally known methods for hollow forming such composite wheel rims (as well as other products having shapes which provide similar problems) require a discontinuity or joint in the fiber reinforcement which significantly weakens the overall structural strength of the product.